Move functions and lambdas from builtin to uf (#7570)

Move functions and lambdas from builtin to uf (#7570)

This is in preparation for adding better native support for handling lambdas in the higher-order extension of the UF theory.

We require that LAMBDA and function types belong to theory UF so that the theory solver is properly notified.

This also splits the utility methods for computing whether a function is "constant" to its own file.

This PR is code move only.